1. Field of the Invention
The present invention relates generally to photolithography systems and more particularly, to techniques for designing features of photolithography systems to improve the resolution and focus of an image that is projected onto a semiconductor wafer.
2. Description of the Related Art
Photolithography is an important part of semiconductor technology. Devices made from semiconductor wafers depend greatly on the resolution and focus of images directed onto selected regions of the wafers. Although much improvement has occurred in the development of photolithography systems that enable the fabrication of smaller and smaller features sizes, photolithography engineers continue to battle defects in resolution as geometries continue to decrease.
For example, many of today""s photolithography systems are now using deep UV wavelengths (i.e., 248 nm) and deep UV photoresists in efforts to better define the image resolution of patterned photoresist. Unfortunately, it has been observed that many resolution defects occur when feature geometries have angled profiles, with respect to horizontal and vertical features. Consequently, when photoresists are developed after being exposed, only horizontal and vertical feature geometries exhibit good resolution, while angled features are substantially distorted.
An example of a photolithography system 100 is shown in FIG. 1A, which includes a scanner system 102. The scanner system 102 is also known as a stepper apparatus. A light source 104 is commonly positioned near a top region of the scanner system 102 in order to allow produced light waves to be directed toward a first lens system 106. From the first lens system 106, the light is projected through a pupil aperture 108 that is used to better direct light onto a second lens system 110. As is well known, the pupil aperture 108 assists in precisely directing the light source onto the desired location of a reticle 112.
The reticle 112 being a glass plate, is patterned with exemplary feature geometries typically defined by a chromium material, which blocks light from propagating through the reticle 112. After the desired light passes through the reticle 112, it leaves the scanner system 102 and comes into contact with a die region 114a of a semiconductor wafer 114 having a photoresist covered surface.
The light then changes the chemical composition of the photoresist so that a developer will allow removal of the exposed regions of photoresist material (i.e., for positive photoresists). In this manner, the feature geometries of the reticle 112 are transferred to the die region 114a. For ease of illustration, only one die region 114a is shown, but as is well known in the art, many more die regions 114a are formed throughout the semiconductor wafer 114 during normal fabrication.
FIG. 1B is a top view of one example of a conventional pupil aperture 108a. The pupil aperture 108a (also known as a clean-up aperture) includes an aperture 116a with a "sgr" value of about 0.6. The pupil aperture 108a is used to more precisely direct light received from the light source 104 onto the reticle 112. Generally, the aperture 116a will define a cone of light that is directed toward the second lens system 110 and then illuminates the reticle 112. Although this pupil aperture 108a assists in more precisely controlling the direction of the light from the light source 104, as demands for smaller and more defined feature resolution continues to increase, the precision provided by the pupil aperture 108a has failed to produce adequate results.
In order to increase resolution of the pattern printed on the die region 114a, several different pupil aperture designs have been devised. FIG. 1C shows an example of an off-axis pupil aperture 108b. The pupil aperture 108b includes a number of off-axis apertures 116b. For purposes of explanation, a zero order region 118 is shown defined around a center point 119 from which an offset 120 measurement is made to the off axis apertures 116b. 
In the pupil aperture 108b, most of the center portion actually blocks the passage of light, thus enabling a focusing of the light that passes through the off-axis apertures 116b. Although the added level of focus precision provided by off-axis apertures is well known, many defects in resolution have still been detected when an off-axis pupil aperture, such as the pupil aperture 108b is used.
FIG. 1D shows an example of a quadrupolar off-axis pupil aperture 108c. The pupil aperture 108c includes a set of four pole apertures 116c, each with a "sgr" value of about 0.1. A horizontal axis 117 is defined through the center point 119 of the pupil aperture 108c. The distance between the center point 119 and the pole apertures is defined by an offset 120. The angle between the pole apertures 116c and the horizontal axis 117 is defined by xcfx86, which is strongly suggested by photolithography scanner equipment manufactures to be exactly 45xc2x0 from the horizontal axis 117.
In fact, scanner equipment manufacturers recommend that when very small feature geometries are being patterned, standard 45xc2x0 quadrupole pupil apertures be used because light received from the first lens system 106 will be more accurately directed to the second lens system 110 and then to the surface of the reticle 112 (as shown in FIG. 1A). Consequently, the scanner equipment manufacturers provide users of their photolithography equipment with standard machined pupil apertures having the aforementioned 45xc2x0 quadrupole design.
In addition, some scanner equipment manufacturers, such as Silicon Valley Group, Inc. (SVG) of Wilton, Conn. provide users of their equipment with guidelines for using the standard 45xc2x0 quadrupole design pupil apertures. Unfortunately, none of the prior art pupil apertures have been able to supply an adequate level of resolution for very small features having angled geometries.
FIG. 1E shows an example of a reticle 112 with a number of feature lines 112b and a corresponding number of angled feature lines 112bxe2x80x2 patterned on the reticle""s glass surface. Also shown are a number of inter-feature spaces 112c defined between any two of the feature lines 112b and its corresponding angled feature lines 112bxe2x80x2. For exemplary purposes, the feature lines 112b/112bxe2x80x2 are patterned such that line widths and spaces as small as 160 nm are transferred onto a resist covered die region 114a as shown in FIG. 1F.
As shown, the die region 114a includes a number of photoresist lines 114b and angled photoresist lines 114bxe2x80x2 that result after development of the exposed photoresist. As evidenced from numerous experimental trials, the photoresist lines 114bxe2x80x2, which have an angled geometric orientation (with respect to a vertical axis), will not produce the ideal pattern shown in the reticle 112.
In fact, because none of the above-described pupil apertures are able to accurately and precisely direct light onto the surface of the reticle 112 when small geometries are being fabricated, major distortion in the developed photoresist will occur as shown in FIG. 1F. It should also be noted that when such distortion occurs, the feature geometries will not produce the desired electrical interconnections, thereby producing a malfunctioning integrated circuit device. Of course, when such malfunctions occur, semiconductor devices are scrapped, and corresponding fabrication yield will suffer.
In view of the foregoing, there is a need for photolithography scanner pupil apertures that assist in more accurately directing light onto a reticle when features having very small critical dimensions are being patterned. There is also a need for methods for manufacturing custom pupil apertures to correct resolution distortions when features having small angled geometries are patterned over photoresist covered wafers.
Broadly speaking, the present invention fills these needs by providing an apparatus, and method for making an off-axis pupil aperture for use in a photolithography system. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device or a method. Several inventive embodiments of the present invention are described below.
In one embodiment, a pupil aperture for use in a photolithography scanner system is disclosed. The pupil aperture includes a plate having a set of pole apertures that are radially offset from a reference center point of the plate. The plate further includes a horizontal reference line that intersects the reference center point. The horizontal reference line is used to define a target angle that is between about 15 degrees and about 35 degrees from the horizontal reference line. The target angle defines an off-axis location for each of the set of pole apertures. In a specific aspect of this embodiment, a set of 4 or 8 pole apertures can be defined in the plate, and their offset from the center point can be selected to be between about 0.3 inches and about 0.9 inches.
In another embodiment, a pupil aperture for use in a photolithography scanner system is disclosed. The pupil aperture includes a plate having a center region that is semi-transparent. The center region has a set of pole apertures that are radially offset from a reference center point of the center region. The plate further includes a horizontal reference line that intersects the reference center point. The horizontal reference line is used to define a target angle that is between about 15 degrees and about 35 degrees from the horizontal reference line. The target angle defines an off-axis location for each of the set of pole apertures. In a specific aspect of this embodiment, the sigma value for each of the pole apertures can be selected to be between about 0.05 and about 0.15. In a more preferred aspect of this embodiment, a set of 4 or 8 pole apertures can be defined in the plate.
In yet another embodiment, a method for making a pupil aperture for use in a photolithography scanner system is disclosed. The method includes machining a plate having a set of pole apertures that are radially offset from a reference center point of the plate, where the plate has a horizontal reference line that intersects the reference center point. The horizontal reference line is used to define a target angle that is between about 15 degrees and about 35 degrees from the horizontal reference line. The target angle defines an off-axis location for each of the set of pole apertures.
In yet another embodiment, a pupil aperture is disclosed. The pupil aperture includes a disc means having a set of pole aperture means that are radially offset from a reference center point of the disc means. The disc means has a horizontal reference line that intersects the reference center point. The horizontal reference line is used to define a target angle that is between about 15 degrees and about 35 degrees from the horizontal reference line. The target angle defining an off-axis location for each of the set of pole aperture means. Most preferably, the target angle is selected to be about 22.5 degrees from the horizontal reference line.
One advantage of the present invention is that it allows light to be more precisely directed onto a photoresist covered semiconductor wafer. Thus, any major distortions resulting from stray light is eliminated and the pupil aperture is able to improve the resolution of the features being patterned from the reticle to the photoresist. This is particularly powerful when features having very small critical dimensions and angled geometries are being patterned. Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.